I think the welded hulls & the fiberglass hulls are about the same weight. I've had both in the 12' range w/4 cyl. Lycoming and they both performed about the same on the hill.
Here is a link to some info I found out by calling some manufacturers.
The only aluminum that was even close to being as light was really thin...check out the Predator Hulls...they are fiberglass, light, and STRONG!!!. The builder is a member and has some pictures of them in his gallery.
( The photo of him jumping the boat is impressive but he did it even higher later that day I hear....)